The present invention relates to the manufacture of magnetic recording media. More particularly, the present invention relates to a method for manufacturing a magnetic recording medium having a very thin magnetic layer not exceeding 1.0 .mu.m in thickness as measured in a dry state. The present invention provides a method for manufacturing a magnetic recording medium having very excellent orientation of magnetic particles and very favorable electromagnetic conversion characteristics.
It is conventional to manufacture magnetic recording media, such as video tapes, audio tapes, and the like, by applying a coating liquid containing particles of a ferromagnetic material dispersed in a bonding agent dissolved in an organic solvent on a non-magnetic supporting base (web). The supporting base has a belt shape and is continually transported in the longitudinal direction thereof. After applying the coating liquid, the coating liquid is dried and solidified. Thereafter, the supporting base is cut to produce the magnetic recording medium.
However, it is necessary in the production of magnetic recording media to increase the rectangular ratio (i.e., the value obtained by dividing the residual flux density B.sub.r by the saturated or maximum flux density B.sub.s) of the magnetic coating, with the magnetic particles being oriented in the transport direction of the non-magnetic supporting base. By increasing the rectangular ratio, the magnetic sensitivity of the magnetic media is increased and the signal-to-noise (S/N) ratio achieved on the media can be improved. For this reason, it has been known to apply a process of arranging (orienting) the direction of the axis of easy magnetization of the magnetic particles towards the transport direction of the non-magnetic supporting base by exposing the base to a magnetic field in the transport direction by means of a permanent magnet or a solenoid coil or the like while the coated liquid is still wet.
In recent years, there has been a strong demand for an increase in the storage capacity of recording media. In order to attain an increase in capacity, the information recording density per unit area on each recording medium must be increased. At the same time, it is necessary to reduce the area in which a writing signal is concentrated in the form of magnetic flux generated by a magnetic head. As a result, the size of the magnetic head and also the amount of magnetic flux generated must be reduced. In addition, the volume of the magnetic recording layer which can be processed for a reversal of magnetization must be reduced so that it will not be possible to make any complete reversal of magnetization unless the thickness of the magnetic recording layer is reduced.
Thus, in order to increase the recording density of a recording medium, it has become necessary to form a magnetic recording layer with a thinner layer to achieve an increase in storage capacity.
However, in the case of a thin layer magnetic coating with a dry layer thickness not exceeding 2 .mu.m, the surface area of the magnetic particles is large compared with the amount (volume) of the magnetic coating liquid. Thus, the ratio of decrease due to the evaporation of the solvent in the magnetic coating liquid is increased and the viscosity of the coating increased accordingly. As the evaporation occurs before a magnetic field is applied to properly orientate the magnetic particles in the medium, it is more difficult for the magnetic particles to move into the proper alignment. Thus, it has been found that the rectangular ratio will not be improved to any significant extent even if the above-mentioned magnetic field is used to orient the magnetic particles. Furthermore, in dual-layer coating process in which an upper magnetic layer is coated on the lower layer previously dried, it has been noted that the magnetic particles of the upper layer will have still greater difficulty in their movement due to an increase in the viscosity of the coating liquid. This increase is brought about by the absorption of the solvent of the magnetic coating liquid into the lower layer of the magnetic recording medium.
A method of overcoming the above problems has been proposed in Japanese Laid-Open Patent Applicant No. Sho. 62- 92132. This disclosure describes a method for manufacturing a magnetic recording medium that includes the steps of providing both a thin magnetic layer and a non-magnetic undercoat layer at the same time using a simultaneous dual-layer coating process and applying a magnetic field to both of the layers while they are still in their undried states.
Nevertheless, the combination of the thin magnetic layer and the lower non-magnetic layer as proposed above does not produce any notable improvement on the rectangular ratio even if a magnetic field is applied to the two layers.